I know only a little about these matters, but perhaps I can get you started. The small Danfoss compressors used for most marine fridges are used for all sorts of fridges. Danfoss sells controller boards that plug into the compressors; some take 12v DC input, others 120/240v AC, and there are boards that can be powered by either AC or DC (and preferring AC but switching to DC when AC is unavailable). Some fridge manufacturers use their own controller designs, particularly when they add extra bells and whistles. So if you want to convert a fridge with a Danfoss compressor from AC to DC (or both) then find the exact model of compressor used then go to the Danfoss site and look for compatible controllers - if your lucky there may be a 12v model for your compressor. A new controller board should run about $200 IIRC, from rparts.com. The controller board is plugged directly to the sealed compressor. OTOH you could just get a large enough inverter to power the fridge as is and live with the loss of efficiency.

The fridge you show would be inefficient with the front doors, and it may not be too well insulated, so there are other issues to think about.

If you decide to buy a marine 12v ice box conversion, then I would recommend going the simple, inexpensive route with standard Danfoss controller and compressor and skip the fancy (energy-robbing) controllers others offer. Others here will disagree...

Some Danfoss controllers support automatic selection of compressor speed for a modest efficiency improvement. I think all models support manual speed selection, which is useful and easy to wire.

I believe it is slightly more complicated, than just a controller board. Shouldn't the compressor motor voltage match the input too? The way it was explained to me once was, that for a boat application one would ideally have a 12V DC compressor with attached 12V DC control board, and use additional 230V(110V)AC/12VDC power supply for when at shore power.

What I am putting to builders of the fridge is thicker insulation in the first cabinet and call it a freezer
And instead of installing 240volt danfoss units, put in 12 volt units at the start.
Even if I have to put in 2 units, I would imagine it will still work out more affordable than what the competition is selling

As for cold air spilling out, I reckon ease of use is more important to us and I have seen tests with electronic thermometers in front loaders where the temp does drop marginally when the door is opened, but it is back down to the original temp in probably less than a minute when the door is shut.
Plus I don't have to take half the contents out putting it all over the bench top while searching for that jar of whatever.

MRM- As I said,I don't know too much about it :-) I just checked the Danfoss site and you are right: the household compressors themselves run on AC and the mobile ones DC.

So if one wants an AC/DC system using Danfoss components it is necessary to start with a DC compressor and add the multi-voltage controller. But it appears worse than that: the household units are spec'ed for either 50Hz or 60Hz, but not both. So using a transformer to make a 240V/50Hz system work on 120V/60Hz may not be adequate (if you plan on visiting the US, Japan, or others w/120V/60Hz). A large inverter, preferably sine wave, would solve the problem. Combined with a large batterycharger (multi-voltage/freq) that could work. It would make me nervous depending on all of that working, though: too many things could fail.

Very good point. I installed an Isotherm unit while in the Caribbean, at the time that Isotherm was still located in Kalmar, Sweden. The first units of the new design to arrive in the Caribbean weren't up to the task.

I have been through this dilemma with my motorhome conversion. After a few years of trials and more recent trials with LYP/LFP batteries a modern 240v unit powered via an inverter wins hands down. In Australia there is a star rating for electricalappliances, the testing for these is extreme. To even qualify for testing the fridge/freezer has to be able to pull the freezer down to -15degC and fridge to 5degC from a heat soaked temp of 40degC within 15hrs with no artificial airflow across the condenser. No 12v fridge would pass this test. Comparative side by side testing of two 250ltr fridge freezers used every day, the 240vac unit was more efficient even with the inverter load that the low voltage purpose built unit. The other pluses are,Auto defrost If anything needs repairing it's much easier to find a domestic refrigeratormechanic than it is to find a low voltage refrigeratormechanic.

To even qualify for testing the fridge/freezer has to be able to pull the freezer down to -15degC and fridge to 5degC from a heat soaked temp of 40degC within 15hrs with no artificial airflow across the condenser. No 12v fridge would pass this test.

Hardly surprising. This test tells only, how powerful (kW) were the compressors motors. One would need a huge (expensive) battery bank to
try to compete in this field. However, feeding all that power from batteries to a powerful AC unit via inverter needs even more capacity... Not very fair criteria IMO.

Quote:

Comparative side by side testing of two 250ltr fridge freezers used every day, the 240vac unit was more efficient even with the inverter load that the low voltage purpose built unit.

Considering high efficiency of DC motors this is interesting. Were they two otherwise *identical* units differing only in compressors used? Do you have links to this test?

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The other pluses are, Auto

Quote:

defrost. If anything needs repairing it's much easier to find a domestic refrigerator mechanic than it is to find a low voltage refrigerator mechanic.

What is so unique about auto defrost and AC?
Depending on locale you may be right about repairs.

Keep in mind that none of the small Danfoss DC compressors are really DC, they simply have DC controllers. The compressors themselves are 3 phase AC. 3 phase AC motors are the most efficient electric motors available. They are used because even after conversion losses in the controller they still use less power than an equivalent output DC or single phase AC motor. They are also smaller and lighter and are easier to make multispeed. I lost a controller in my fridge last year and it cost $240. Three years ago the one in my home fridge fried, $240. I think I may be seeing a trend.

My understanding is that the smaller Danfoss compressors are able to run at significant degrees of incline whereas the cheaper compressors used in home type systems must be level. Combine that with lower production volumes and you get higher prices. The OP obviously has a cat which reduces the motion tolerance requirement, but it does not eliminate it. As far as I can tell pretty much all of the small mobile unit builders use Danfoss compressors, which pretty much means that Danfoss can charge about anything they want for their compressors. I guess we're lucky it's not worse.

There is a reason that Danfoss rules the roost: they make a better product. My original Isotherm had a GE compressor (the US company General Electric). In less than a year I paid to have it swapped out as the newer design worked better in the Caribbean (there had been a lot of complaints). Then a couple of years later that compressor failed during a bouncy passage around Beachy Head on the south coast of England. Finally the replacement unit failed a few years later in Turkey, having been operated for the previous 4 years in a marina in Turkey (i.e. no movement to speak of). So that is 3 compressors in 10 years, 2 of which failed completely. There is a reason Isotherm switched to Danfoss.

In Turkey I installed a simple Danfoss system and it has worked very well since then. The compressors and controllers are readily available, and are the standard for the marine industry. So no complaints about Danfoss...

That unit in the original post draws 2.5 amps @ 240 volts AC. That translates into an inverter draw of about 60 amps on your 12V batteries, allowing for only 25% loss.

Quote:

Originally Posted by btrayfors

Do you really want a frig that draws that much? CF the modern 12V friges which draw 4-6 amps.

Bill

That must be one big compressor, are you sure you aren't reading the stall current and translating that as run current. My Fisher & Paykel 250ltr fridge/freezer draws 80ah per 24hr period including the 9% inverter losses, a slightly smaller sized Waeco 212ltr fridge freezer without auto defrost used 82ah for the same 24hr period, side by side, both used as a party beer fridge including ice for the ladies drinks. If you are using an inverter with a 25% loss, how do you keep it cool? The compressor run current for the 250ltr Fisher & Paykel is 7amps @ 12v, this includes the inverter 1amp operating current. If lithium iron batteries are used there is no voltage drop at anything under 0.5CA ( an amp draw comparable with 50% of the battery capacity, i.e. 50 amps for a 100ah battery) and will deliver 80% of their capacity and still maintain 13v at terminals along with a true rating output, Peukert’s factor doesn't come into it with lithium iron batteries as long as the draw is under 1CA.Low voltage appliances and lead acid batteries are stuff left over from last century, not stuff for new fitouts. I can post a cell logger chart of 2 x 250ltr 240vac fridges running of one lithium iron battery via an inverter for 36hr with no form of recharging if that will help convince you.

Talking to other cruisers that have built their own cooling systems Danfoss seems to be the most common. I work with refer systems on dry cleaning machines. We use scroll compressors. These compressors are very efficient and are able to take a slug of liquid Freon. On a boat that would seem to be very important. Problem is most scroll compressors are large units and 220volt. Combing the internet I am unable to find a 12v dc unit. As the previose posting mentioned a proper controller will solve the 12v problem. Most auto compressors are scroll. These compress will work with your engine.